1. Field of the Invention
The present invention relates to a vehicle height adjustment system for adjusting the vehicle height of a vehicle.
2. Description of the Related Art
Many large vehicles such as a heavy-duty truck have so-called air suspensions, which use air springs as vehicle chassis springs.
Trucks or other large vehicles can secure a given suspension stroke irrespective of the weight of loaded cargos, by keeping the height of a body frame (vehicle height) at a predetermined height from an axle. Accordingly, it is possible to improve the ride comfort and also to reduce the vibration of a cargo bed effectively. The “predetermined height” is called a reference vehicle height and the value thereof is suitably determined depending on the vehicle model.
Trucks or other large vehicles having an air suspension, in particular, often include a vehicle height adjustment system which is capable of adjusting the vehicle height by supplying and discharging air to/from air springs. Specifically, this vehicle height adjustment system first determines a current vehicle height, for example, by detecting a distance between an axle and a body frame, and the like, and then compares the current vehicle height with a predetermined reference vehicle height. When the current vehicle height is different from the predetermined reference vehicle height, the vehicle height adjustment system performs such adjustment as to maintain a constant vehicle height by supplying air to air springs of the air suspension from an air tank or air bellows or by discharging air from the air springs to the atmosphere (for example, see the Patent Document 1).
(Patent Document 1) Japanese Patent Laid-Open No. H10(1998)-324131
However, if no limit is imposed on a vehicle height adjustment range, unnecessary supply and discharge of air are performed. Therefore, an upper limit and a lower limit are normally given for the vehicle adjustment. As a method of setting these limits, the vehicle height is adjusted, while being visually monitored, by supplying or discharging air to/from air springs, and then, when there is no change in a value of the vehicle height for a given period of time, this value of the vehicle height is set as an upper limit or lower limit.
Generally, a stopper and a stopper receiving portion are provided on a body frame side and an under member side to which an air spring is fixed, respectively. When air is discharged from the air spring and the body frame is lowered, the relative distance between the stopper and the stopper receiving portion is gradually reduced. The stopper comes into contact with the stopper receiving portion before the air in the air spring is completely bled off, and thus the body frame is prevented from being further lowered. With such a structure, the load of the lowered body frame side is borne by an axle housing, with the stopper and stopper receiving portion mainly made from an elastic member and interposed between the body frame and the axle housing. In this way, the body frame does not come into direct contact with the axle housing when the air in the air spring is discharged and the bearing power by means of air-pressure is significantly reduced. When the air is gradually discharged, the stopper first comes into contact with the stopper receiving portion. Thereafter, the vehicle height is gradually lowered while the stopper or stopper receiving portion is compressed, and at a certain level, the vehicle height is not lowered any further. Thus, the vehicle height at this level is made to be the lower limit. In such a case, if the vehicle height is intended to be lowered to the lower limit, the discharge of air from the air spring is continued even after the stopper has come into contact with the stopper receiving portion, up until the stopper or stopper receiving portion is compressed and the vehicle height reaches the lower limit, performing excessive air discharge. Accordingly, when the supply of air to the air spring is started in order to raise the vehicle height again, the vehicle height is not raised immediately, consuming time to separate the stopper from the stopper receiving portion. That is, the vehicle height adjustment time is increased, and the amount of air consumption becomes large as well.
Such disadvantages are avoidable by stopping the air discharge before the body frame reaches the vehicle height lowest position. However, it is difficult to uniformly set vehicle heights where the air discharge is stopped, because of assembly errors of a height sensor, air spring, stopper, stopper receiving portion, and the like. If the vehicle heights where the air discharge is stopped are uniformly set high in order to avoid the influence of the assembly errors, the vehicle height adjustment range is unnecessarily narrowed. In addition, the above-described method of setting the upper and lower limits also has a problem that the upper and lower limits need to be set individually.